Flow past a slender body of revolution at high angle of attack separates from the leeward side of the body and rolls up into a pair of vortices. The aerodynamic forces imparted by the vortices enhance the lift of the vehicle. Above some critical angle, the vortices may become asymmetric, leading to unpredictable side forces that can exceed the aerodynamic vehicle's control capability, resulting in a departure from controlled flight.
Studies have shown that vortex asymmetry at high angles of attack is triggered by minute imperfections, or micro-asymmetries, located at the nose tip of the slender body. Numerous techniques have been used to actively control forebody vortex asymmetry, including suction/blowing (Yuan, Staudacher), strakes (Murri, Walters, Staudacher, Rao), and inflatable membranes (Zell), to varying degrees of success.
To be a viable solution, the system must have no ill performance effects at flight regimes where it is not needed. Thus, many solutions employ some sort of retractable feature so that they can be deployed as needed. One of the biggest challenges that arise is that the volume near the tip of a slender forebody is limited, and active vortex control systems that employ conventional mechanical or pneumatic action to deploy the system can take too much space to be of practical use.